This invention relates generally to the field of blood pressure monitoring methods and devices and more particularly to auscultatory blood pressure monitoring methods and devices employing means for removing noise and vibration effects from audible heart beat sounds.
The blood pressure in the brachial artery is not constant, but varies with time in relation to the beating of the heart. Following a contraction of the heart to pump blood through the circulatory system, the blood pressure increases to a maximum level known as the systolic blood pressure. The minimum blood pressure between heartbeats is known as the diastolic blood pressure.
The traditional technique for measuring the blood pressure of a patient employs an inflatable pressure cuff wrapped around an upper arm of a patient whose blood pressure is to be determined. As the pressure cuff is inflated, cuff pressure and pressure applied to the arm of the patient increases. If the pressure applied to the arm is increased beyond the highest blood pressure in the brachial artery located in the arm beneath the pressure cuff, the artery will be forced to close.
As the pressure in the inflatable cuff is reduced from a high level above the systolic blood pressure, where the brachial artery is permanently closed, to a level below the systolic blood pressure level, the brachial artery beneath the cuff will begin to open and close with each heart beat as the blood pressure first exceeds the cuff pressure and then falls below the cuff pressure. As the blood pressure exceeds the cuff pressure, the artery will open, and a low frequency blood pressure sound corresponding to the heart beat can be detected. This sound is detected using a stethoscope or microphone placed near the down-stream end of the cuff on the patient""s arm. The highest cuff pressure at which the heart beat sounds are detectable thus corresponds to the systolic blood pressure of the patient.
As the cuff pressure is reduced further, the cuff pressure will be brought below the diastolic blood pressure. At this pressure level, the brachial artery beneath the cuff remains open throughout the heart beat cycle. Blood pressure sounds, caused by the opening of the artery will, therefore, not be produced. The lowest cuff pressure at which the blood pressure sounds can be detected thus corresponds to the diastolic blood pressure of the patient. The determination of blood pressure based on the detection of the onset and disappearance of blood pressure sounds as varying pressures are applied to an artery, is known as auscultatory blood pressure determination.
In manual auscultatory blood pressure measurement methods, a stethoscope is used to detect the onset and disappearance of the blood pressure sounds. Thus, the blood pressure measurement is highly dependent on the skill and hearing ability of the person taking the measurement. To overcome this dependence on human skill and judgement, and to automate the process of determining a patient""s blood pressure, automatic blood pressure monitoring systems based on the auscultatory method of blood pressure determination have been developed. These automatic systems employ one or more microphones placed in or under an inflatable cuff to detect blood pressure sounds.
However, it is almost impossible to detect the blood pressure sounds in a noisy environment such as a moving ambulance, helicopter, airplanes, or naval vessels.
Pneumatic systems measuring pressure variations caused by blood flowing through the artery instead of sound are not sensitive to noise, but extremely sensitive to movement and vibrations. Pressure variations caused by patient movement and any vibrations present are generally much larger than the pressure variations by the blood flow thus rendering these systems useless in the environments mentioned above.
Some blood pressure monitoring systems employ two microphones for detecting blood pressure sounds. For example, two microphones may be placed under the inflatable cuff separated by a distance such that a low frequency blood pressure sound will reach the first microphone 180 degrees out of phase from the second microphone. Noise signals will tend to reach each microphone essentially simultaneously, and in phase. Therefore, subtracting the two microphone signals from each other will tend to enhance the useful data and diminish unwanted noise. The two microphone signals can be added and subtracted from each other to create signal and noise detection thresholds. Microphone signals are considered to be valid blood pressure sound detections if they meet the detection thresholds. These blood pressure monitoring methods tend obtain useful data in moderately noisy environments. However, these systems are less effective when confronted with significant noise levels.
In U.S. Pat. No. 5,680,868 issued to Kahn et al. in Oct. 28, 1997 a method and apparatus for monitoring the blood pressure of a patient by detecting low frequency blood pressure sounds in the presence of significant noise levels is disclosed. Kahn discloses two microphones placed over the brachial artery of a patient to detect the onset and disappearance of blood pressure sounds in the artery as the pressure on the artery is varied. The microphones are placed on the patient separated by a distance such that a true blood pressure sound will preferably be picked up at the second microphone approximately 180 degrees out of phase with respect to the blood pressure sound picked up by the first microphone. The shift in phase between the signals from the two microphones is used to indicate the detection of a blood pressure sound in the presence of significant noise levels. However, the phase detection method is still affected by vibrations detected out of phase at the two microphones. This method is based on the assumption that noise and vibrations are detected at both microphones without a phase shift whereas the blood pressure sound has a phase shift of approximately 180 degrees. Vibrations due to body motion such as shivering or ambient vibrations imposed on the body will generally be detected out of phase at the two microphones making it difficult to detect the beginning and end of a blood pressure sound signal as the pressure cuff deflates. Furthermore, this method requires an extensive amount of computation making it difficult to manufacture a portable device using this method. Another disadvantage of this method is that it is not possible to obtain directly from the processed signals a heart rate, which provides live saving information in emergency situations.
It is an object of the invention to provide a method and a device for measuring systolic and diastolic blood pressure in environments comprising extreme levels of noise and vibration, which overcomes the aforementioned problems.
It is further an object of the invention to provide a method and a device for measuring systolic and diastolic blood pressure in environments comprising extreme levels of noise and vibration that also provides information about the heart rate.
It is yet another object of the invention to provide a method and a device for measuring systolic and diastolic blood pressure in environments comprising extreme levels of noise and vibration enabling accurate measurement of blood pressure during low flow states, such as cardiogenic shock.
It is yet another object of the invention to provide a device for measuring blood pressure in environments comprising extreme levels of noise and vibration that is battery operated and portable.
In accordance with the present invention there is provided, a method and device for measuring systolic and diastolic blood pressure and heart rate in environments with extreme levels of noise and vibrations. Sensing only noise and vibrations and subtracting it from a measured blood pressure signal using an adaptive interferer canceller provides good results even under extreme conditions such as aboard a helicopter.
In accordance with the present invention there is provided, a method for measuring systolic and diastolic blood pressure of a patient comprising the steps of:
sensing blood pressure signals corresponding to heart beat using a first acoustic sensor placed on the patient near a location of an artery of the patient, the first acoustic sensor for producing a first acoustic signal in dependence upon the blood pressure signals;
sensing noise and vibrations using a second acoustic sensor placed on the patient at a location away from an artery, the second sensor for producing a second acoustic signal in dependence upon noise and vibrations;
sensing pressure applied to the artery using a pressure transducer for sensing pressure and for providing a pressure signal in dependence upon the sensed pressure;
providing the first acoustic signal, the second acoustic signal and the pressure signal to a processing unit while the pressure is applied to the artery;
processing the first acoustic signal for removing interference due to noise and vibrations in the first acoustic signal by subtracting the second acoustic signal from the first acoustic signal using an adaptive interferer canceller algorithm;
detecting heart beat pulses within the processed first acoustic signal; and, determining systolic and diastolic pressure by relating the detected heart beat pulses to the pressure signal.
In accordance with another aspect of the present invention there is provided, a method for monitoring heart beat of a patient comprising the steps of:
sensing blood pressure signals corresponding to heart beat using a first acoustic sensor placed on the patient near a location of an artery of the patient, the first acoustic sensor for producing a first acoustic signal in dependence upon the blood pressure signals;
sensing noise and vibrations using a second acoustic sensor placed on the patient at a location away from an artery, the second sensor for producing a second acoustic signal in dependence upon noise and vibrations;
providing the first acoustic signal and the second acoustic signal to a processing unit;
removing interference due to noise and vibrations in the first acoustic signal by subtracting the second acoustic signal from the first acoustic signal using an adaptive interferer canceller algorithm;
detecting heart beat pulses within the first acoustic signal; and,
determining a heart rate from the detected heart beat pulses.
In accordance with the present invention there is further provided, a device for measuring systolic and diastolic blood pressure of a patient in an environment with extreme levels of noise and vibration, the device comprising:
a pressure cuff for applying pressure to an artery of the patient;
a pressure transducer for providing a pressure signal in dependence upon the pressure applied to the artery;
a first acoustic sensor for producing a first acoustic signal in dependence upon blood pressure signals corresponding to heart beat;
a second acoustic sensor for producing a second acoustic signal in dependence upon noise and vibration; and,
a processing unit for processing the first acoustic signal using the second acoustic signal in an adaptive interference canceller algorithm, for detecting heart beat pulses within the processed first acoustic signal and for determining systolic and diastolic blood pressure using the detected heart beat pulses and the pressure signal.